Processes for oxide based phosphors

ABSTRACT

A phosphor comprises, in atomic percentages, 90% to 100% of a mixed metal oxide MxTyOz, wherein M is a metal selected from Zn, Sn, In, Cu, and combinations thereof, T is a refractory metal selected from Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, and combinations thereof, and O is Oxygen, x, y, and z being chosen such that z is at most stoichiometric for MxTyOz; and 0% to 10% of a dopant comprising a substance selected from a rare earth element of the lanthanide series, Mn, Cr, and combinations thereof, or stoichiometrically excess zinc, copper, tin, or indium. Cathodoluminescent phosphor compositions stimulable by electrons of very low energy are prepared from metal oxides treated with refractory metals in various processes disclosed. Metal oxides or mixed-metal oxides of zinc, copper, tin, or indium are heated in the presence of a refractory metal such as titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, or combinations or alloys thereof to make phosphors of various chromaticities. In a simple embodiment, a quantity of Ta 2  O 5  is added to a quantity of ZnO and heated at an effective temperature and time to form Ta 2  Zn 3  O 8 , which is useful in various forms as a blue-light-emitting phosphor. In preferred embodiments, the phosphors are prepared in situ in an electrically-conductive thin-film or surface-layer form during fabrication of displays.

RELATED PATENT APPLICATIONS

This application is related to the following U.S. Provisional PatentApplications of Michael D. Potter: Ser. No. 60/025,550 titled "NewPhosphor and Synthesis" filed Sep. 3, 1996; Ser. No. 60/025,555 titled"Integrated Etch Stop and Phosphor Process" filed Sep. 3, 1996; Ser. No.60/025,556 titled "Integrated Etch Stop and Phosphor Process for FieldEmission Device Display Applications" filed Sep. 3, 1996; Ser. No.60/032,197 titled "Integrated Etch Stop and Low Voltage Phosphor Processfor Field Emission Device Display Applications" filed Dec. 2, 1996; Ser.No. 60/032,199 titled "Integrated Etch Stop and Low Voltage PhosphorProcess" filed Dec. 2, 1996; and Ser. No. 60/032,201 titled "New LowVoltage Phosphor and Synthesis" filed Dec. 2, 1996. This application isalso related to three co-pending U.S. Patent Applications of Michael D.Potter titled "Oxide Based Phosphors and Processes Therefor," "ElectronField-Emission Display," and "Fabrication Process for ElectronField-Emission Display," each filed on the same date as thisapplication, Jul. 28, 1997.

FIELD OF THE INVENTION

This invention relates generally to phosphors and more particularly tonew oxide-based phosphors particularly useful as cathodoluminescentphosphors excited by low-energy electrons, and processes for preparingthe new phosphors, including processes for preparing the phosphors insitu while fabricating a display, such as an electron field emissiondevice display (FED) or a vacuum fluorescent display (VFD).

BACKGROUND OF THE INVENTION

A phosphor emitting light in response to stimulation is useful in manytechnical fields, including fluorescent lights, electron field-emissiondevice displays (FED), and vacuum fluorescent displays (VFD). A phosphorresponsive to excitation by electrons of low energy (i.e. accelerated bya low voltage) is particularly useful, and there are particular needsfor a blue-light-emitting phosphor of high spectral purity. The hue oflight emission from a phosphor is often described in terms of awavelength or range of wavelengths of emitted light, such as thewavelength of a major or dominant peak in the phosphor's emissionspectrum, or by coordinates (x and y) in a CIE (CommissionInternationale d'Eclairage) chromaticity diagram. Blue light isconventionally characterized in the wavelength sense by a spectrum witha dominant peak between about 380 nanometers and about 480 nanometers,e.g. around 430 nanometers (nm). Chromaticity x and y values in theregion of the CIE 1931 chromaticity diagram corresponding to blue huesare in a region about x=0.15, y=0.1. Representative blue primaries invarious standards for RGB display systems correspond to similar CIE 1931chromaticity x and y values: for example, CIE spectrum primary bluex=0.167, y=0.009; NTSC standard primary blue x=0.140, y=0.080; andgraphics-monitor primary blue x=0.150, y=0.070.

There is a long-standing and continuing need for phosphors emitting inthe blue region of the spectrum with high spectral purity. Phosphorssuch as ZnO:Zn, ZnS:Au, CdWO₄, and Zn₂ WO₅, (each having blue-green peaklight emission) and ZnGa₂ O₄, ZnS:Zn, and ZnS:Ag (each having generallyblue peak light emission) have been known in the art for many years.(See, for example, the article by H. W. Leverenz, "Phosphors Versus thePeriodic System of the Elements" Proc. I. R. E. [May 1944] pp. 256-263.)U.S. Pat. No. 4,275,333 to Kagami et al. describes fluorescentcompositions and low-velocity-electron excited fluorescent displaydevices, utilizing phosphors which include blue-light-emittingphosphors. U.S. Pat. Nos. 5,120,619 and 5,250,366 to Nakajima et al.describe rare earth tantalate and/or niobate phosphors which emit lightunder X-ray excitation, with peak light emission generally below 370 nm,and typically between about 320 nm and 350 nm. U.S. Pat. No. 5,138,171to Tecotzky et al. describes a photostimulable X-ray energy absorbinghalosilicate, halogermanate, or halo(silicate-germanate) phosphor havingprompt light emission with a peak wavelength around 445 nm. U.S. Pat.No. 5,478,499 to Satoh et al. describes a low-velocity electron excitedphosphor of blue luminous color having CIE 1931 chromaticity diagram yvalues of 0.05 to 0.25. U.S. Pat. No. 5,507,976 to Bringley et al.describes stabilized phosphor intermediates and storage phosphorscapable of storing latent X-ray images for later release. At least someof the storage phosphors taught by the Bringley et al. patent are theproducts of firing combinations ("stabilized intermediates") includingoxides with oxosulfur reducing agent for molecular iodine. U.S. Pat. No.5,549,843 to Smith et al. discloses annealed alkaline earth metalfluorohalide storage phosphors including metal oxides. U.S. Pat. No.5,571,451 to Srivastiva et al. describes a quantum-splitting oxidephosphor doped with Pr³⁺, which has an emission spectrum having a peakemission at 400 nm when excited by vacuum ultra-violet radiation.

Many phosphors have been developed with pigments incorporated into,attached, or coated on phosphors to modify the light emitted by theunderlying phosphor in order to achieve a desired hue or a desired colortemperature. For example, U.S. Pat. No. 4,152,483 describes a pigmentcoated phosphor and process for manufacturing it; U.S. Pat. No.4,699,662 to Nakada et al. describes a blue pigmented phosphor; U.S.Pat. No. 5,077,088 to Jeong describes a process for preparation of apigment-coated phosphor; and U.S. Pat. No. 5,363,012 describes apigment-attached blue-emitting phosphor.

U.S. Pat. No. 5,455,489 to Bhargava describes displays comprising dopednanocrystal phosphors which comprise separated particles of a hostcompound activated by a dopant, the phosphor particles being of theorder of 100 angstroms in size and exhibiting quantum-confinedproperties. Examples of such doped nanocrystal phosphors includeZnS:Mn²⁺ (yellow) and ZnS:Tb³⁺ (green), and II-VI host phosphors dopedwith Thulium (Tm), Terbium (Tb), and Europium (Eu) for blue, green, andred light emission respectively.

In an article by Roger T. Williams, Steven R. Evatt, James D. Legg, andMark H. Weichold "Blue light emission observed in a monolithic thin filmedge emission vacuum microelectronic device" Journal of Vacuum Scienceand Technology B, V. 13, No. 2 (Mar./Apr. 1995), p. 500 ff, lightemission at about 488 nanometers wavelength was reported from amulti-layer phosphor structure (Al, ZnO, and W) under forward biasconditions.

Many of the phosphors known in the art are conventionally prepared bymethods including grinding the phosphor composition and/or itsprecursors to a powder having a particle size distribution suitable fora particular purpose. For example, phosphors prepared for X-ray storagepanels may have a median particle size of about 0.5 to 40 micrometers.U.S. Pat. No. 5,536,383 to Tran Van et al. teaches the use of anon-aqueous suspension for the deposition of luminescent materials,particularly phosphors, by electrophoresis. Phosphor powder particles inthe suspension have the finest possible grain size, e.g. approximately 1to 10 micrometers. In a field-emission-excited cathodoluminescencedisplay structure taught by Tran Van et al. the phosphor is deposited byelectrophoresis onto a transparent, conductive coating, e.g. of indiumand tin oxide (ITO), on a transparent insulating substrate. U.S. Pat.No. 5,601,751 to Watkins et al. discloses a manufacturing process forhigh-purity phosphors of small average particle size, exhibitingsufficient luminescent efficiency for utility in field emissiondisplays. In the process of Watkins et al., a precursor mixtureincluding a host lattice material and a dopant starting material ismilled to obtain a sized precursor mixture having an average precursorparticle size less than about 1 micrometer. Particle size growth duringsubsequent heating for infiltration of the dopant into the host latticestructure is held to a minimum, e.g. less than about 100% or preferablyless than about 50%.

Phosphors intended to be used in vacuum fluorescent displays or displaydevices of the cold-cathode field-emission type should not containsubstances that can contaminate the cathode, causing deterioration ofelectron emission. Thus phosphors containing sulfur (S), cadmium (Cd),or cesium (Cs), for example have not found favor for such applications,as those elements can cause contamination in the displays. U.S. Pat. No.5,619,098 to Toki et al. discloses a phosphor free of S and Cd, madefrom compounds of titanium (Ti), alkaline earth metal, and an element ofgroup 13 of the periodic table.

NOTATIONS AND NOMENCLATURE

The term "phosphor" is used throughout this specification and theappended claims in its conventional meaning, to mean a substance capableof luminescence when suitably excited by an energy source (e.g.electromagnetic waves, electrons, or an electric field). Theelectromagnetic radiation emitted may consist of photons havingwavelengths in the visible spectral range. Stimuli suitable forstimulating the emissions of a phosphor include, but are not limited to:electron bombardment (stimulating cathodoluminescence), other incidentphotons (stimulating photoluminescence or fluorescence), specificallyx-ray photons (stimulating x-ray luminescence), and electric fields(stimulating electroluminescence). The term "dopant" is used herein tomean a substance incorporated in a phosphor as an activator orluminescent center, either substitutionally or interstitially withrespect to the crystal lattice of the host substance, or even adsorbedon a surface of the crystal lattice of the host substance. Such dopantsare conventionally denoted by their chemical symbols appended to thechemical formula for the host substance after a colon, e.g. ZnO:Zn, azinc oxide phosphor doped with excess zinc. The term "dopant" can alsoinclude co-activators used, for example, to facilitate charge transfer."Annealing" as used herein means heating for an effective time andtemperature for a particular purpose, e.g. to effect a chemicalreaction, to allow a desired degree of diffusion, etc. The abbreviation"TZO" is sometimes used herein to represent Ta₂ Zn₃ O₈.

OBJECTS AND ADVANTAGES OF THE INVENTION

A major object of the invention is a phosphor which emits blue light ofhigh spectral purity when suitably stimulated. More specific objectsinclude phosphors having dominant wavelengths of emission of about 400nanometers and having CIE 1931 chromaticity coordinates of about x=0.16and about y=0.08 or having CIE 1976 chromaticity coordinates of aboutu'=0.18 and about v'=0.19. Another object is a blue-light-emittingphosphor which has a relatively narrow bandwidth of light emission.Other objects include phosphors which combine these properties withshort persistence and high luminous efficiency. Yet another object is ablue-light-emitting phosphor that does not require addition of a pigmentor a pigmented coating to achieve a desirable hue. A practical object isa phosphor that is useful in displays of several types and in blue ormulti-color light sources. An important object is a cathodoluminescentphosphor stimulable by electrons of very low energy. Related objectsinclude processes for preparing such a phosphor and processes forfabricating display devices utilizing such a phosphor. A more specificobject is a process for preparing such a phosphor in situ whilefabricating display devices. An even more specific object is afabrication process in which synthesis of a phosphor is integrated withprovision of an etch stop for forming an opening of a desired depth.Another specific object is a fabrication process for forming a thinregion of blue-light-emitting phosphor at the surface of an anode in adisplay device operable using cathodoluminescence excited by electronsof very low energy. A related object is a blue-light-emitting phosphorthat may be prepared as an electrically conductive composition. Yetanother object is a new use for tantalum zinc oxide, Ta₂ Zn₃ O₈. Furtherobjects include phosphors of various dominant wavelengths, prepared bymethods similar to the methods disclosed for blue phosphors, andprocesses adapted for manufacturing such phosphors. In this respect,particular objects include red- and green-light-emitting phosphorscompatible with processing of a blue-light-emitting phosphor, and thecorresponding processes for making them. Other objects are processes forconverting commercially available green-light-emitting phosphors to ablue-light-emitting phosphor. Particular further objects are displays(especially electron field-emission displays) comprising a number ofcells formed with display devices, each display device including aphosphor having the desirable properties mentioned above, and processesfor fabricating such displays.

SUMMARY OF THE INVENTION

Blue-light-emitting phosphors of this invention are based on metaloxides treated in various processes with refractory metals to preparephosphor compositions having improved performance as cathodoluminescentphosphors stimulable by electrons of very low energy. In preferredembodiments, the phosphors are prepared in situ in anelectrically-conductive thin-film or surface-layer form duringfabrication of displays. Examples of the processes disclosed use metaloxides or mixed-metal oxides of zinc Zn, copper Cu, tin Sn, or indium Inheated in the presence of a refractory metal such as titanium Ti,zirconium Zr, hafnium Hf, vanadium V, niobium Nb, tantalum Ta, chromiumCr, molybdenum Mo, tungsten W, or combinations or alloys thereof to makephosphors of various chromaticities. Phosphors made in accordance withthe invention may also include dopants, such as manganese, chromium, alanthanide rare earth element, or stoichiometrically excess zinc,copper, tin, or indium. A preferred process integrates an etch stop withthe in situ phosphor process. The etch stop precisely defines the depthof an opening in a field-emission display cell structure utilizing thelow-energy-electron excited blue-light-emitting phosphor.

The simplest process for making a phosphor in aadd dance with theinvention is to add a quantity of tantalum pentoxide (Ta₂ O₅) to aquantity of zinc oxide (ZnO) and to heat or "fire" the combination at aneffective temperature and time to react at least a portion of each ofthe Ta₂ O₅, and ZnO to form Ta₂ Zn₃ O₈, which can then be used invarious forms as a blue-light-emitting phosphor. Processes for preparingphosphors in powder form, solid form for use as a sputtering target, andthin film forms are described in detail hereinbelow. Optionally, thedopants mentioned above may be used singly or in combination with eachother or with other co-activators to modify the chromaticity of thephosphor in any of its powder, solid, or thin-film forms.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of spectral emission measured from a phosphor made inaccordance with a preferred embodiment of the invention.

FIG. 2 is a CIE 1931 chromaticity diagram showing the chromaticity ofcathodoluminescence from a blue-light-emitting phosphor made inaccordance with a preferred embodiment of the invention.

FIG. 3 is a flow chart illustrating a preferred process for preparing apreferred embodiment phosphor.

FIG. 4 is a flow chart illustrating processes for preparation ofalternate forms of phosphors made in accordance with the invention.

FIG. 5 is a graph of cathodoluminescent spectral emissions measured froma starting material phosphor and from a product phosphor made inaccordance with the invention.

FIG. 6 is a flow chart illustrating a preferred process for in situpreparation of the phosphors.

FIGS. 7a, 7b, 7c, 7d, 7e, 7f, 7g, 7h, 7i, 7j and 7k are side elevationcross-sectional views of a display device structure made in accordancewith the invention.

FIG. 8 is a flow chart illustrating a preferred process for fabricatingthe structure of FIG. 7k.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description, reference is made to the severaldrawings, in which corresponding structural elements are denoted by thesame reference numerals, and process steps are denoted by referencessuch as S1, S2, . . . , etc. Numerical order relationships of processstep references do not necessarily imply a time sequence in which stepsmust be performed. Thus step S1 of Example 1 described below, and stepS20 of Example 4 described below are substantially equivalent, and eachis the first step of the respective process example.

The phosphor of this invention, in its most preferred embodiment, iscomprised essentially of zinc, tantalum, and oxygen. It is preferablymade as a thin film phosphor, but alternatively may be made in a powderform or other forms. Other refractory metals, such as but not limited toTi, Zr, Hf, V, Nb, Cr, Mo, W, or combinations or alloys thereof, can beused in place of or in addition to tantalum. It has been discoveredunexpectedly that other metal oxides, such as tin oxide, indium-tinoxide, and copper oxide, may be used instead of zinc oxide in similarpreparation techniques to produce additional new phosphors havinggenerally similar desirable properties, and furthermore the dominantwavelengths of their light emissions may vary with composition. Thephosphors made using tin oxide, indium-tin oxide, or copper oxide arenot simply Sn-activated, In-activated, or Cu-activated phosphors, inwhich tin (or indium or indium-tin or copper) is an additive introducedin minor amounts as an activator of a host phosphor, but instead thesephosphors contain substantial amounts of tin (or indium or indium-tin orcopper). The phosphor material may also include dopants such asmanganese and/or chromium and/or lanthanide elements and/orstoichiometrically excess zinc, tin, indium, or copper. In thin-filmform, these new phosphors have not shown any evidence of charging at anyvoltage and therefore are especially suitable for low excitationvoltages such as those needed for electron field emission devicedisplays (FED) or vacuum fluorescent displays (VFD). Observations (witha dark-adapted, unaided human eye) of a phosphor made in accordance withthe invention have shown an excitation threshold voltage of less than 10volts with a current density of about 10 μA/cm². However, such a visualobservation does not establish a physical limitation of the phosphor.Sensitive measurements using equipment such as photo-multiplier tubesmay be used to determine a more accurate low excitation voltagethreshold for a particular embodiment of the phosphor.

The cathodoluminescent spectral response of the new phosphor preferredbase material has a dominant peak near 400 nanometers, and a minor peakat somewhat longer wavelengths. FIG. 1 shows the shape of an emissionpeak measured for one embodiment of the phosphor. In FIG. 1, the peakcathodoluminescent emission occurs at a wavelength of about 400nanometers. Effects of the various preparation processes and treatmentsare described in more detail hereinbelow.

FIG. 2 is a CIE 1931 chromaticity diagram showing the chromaticity ofcathodoluminescence from a phosphor made in accordance with a preferredembodiment of the invention. This phosphor has CIE 1931 chromaticitycoordinates of about x=0.16 and about y=0.08, equivalent to CIE 1976chromaticity coordinates of about u'=0.18 and about v'=0.19.

The new phosphor material can be prepared in a number of ways. Forexample, starting materials can be separately deposited and subsequentlyannealed to form the new phosphor. The starting material may beprecursors of the constituents. For example, instead of starting withzinc oxide, the starting material may be a compound capable of beingconverted to zinc oxide by heat treatment. Starting material depositiontechniques include, but are not limited to: sequential sputtering,co-sputtering, sequential evaporation, co-evaporation, molecular beamepitaxy (MBE), ion-assisted epitaxy, atomic layer epitaxy, laserablation deposition, chemical vapor deposition (CVD), metal-organicchemical vapor deposition, electron cyclotron resonance chemical vapordeposition, and plasma-enhanced chemical vapor deposition. The inventionwill be further clarified by considering the following examples, whichare intended to be purely exemplary of the practice of the invention. Apreferred method of preparing a preferred embodiment of the phosphors isdescribed below as Example 1. Other alternative preparation methods aredescribed as Examples 2, 3, and 4.

EXAMPLE 1

Referring now to FIGS. 3 and 7a-7k: a suitable substrate 20 is firstprovided (S1). On substrate 20, a layer of zinc oxide (ZnO) 30 isdeposited (S2) by sputtering, such as of sputtering. ZnO layer 30 may beabout 0.5 micrometer thick. The ZnO may have an excess of zinc, whichforms the "super-stoichiometric" zinc oxide phosphor (ZnO:Zn) known inthe art. The zinc oxide material deposited in this manner tends to formcolumnar crystallites having a c-axis orientation (perpendicular to thesubstrate surface). It has been found by investigation of the newphosphor described herein that such an orientation greatly reduces thinfilm photon refractive trapping. This is a very useful result and ishighly desirable for electronic display applications.

A tantalum layer 40 is deposited (S3) on top of the zinc oxide layer 30.The tantalum can be sputtered, for example. The thickness of thetantalum layer 40 is controlled to provide an effective amount oftantalum, based on the thickness of zinc oxide deposited in step S2. Iftoo much tantalum is used, metallic Ta may remain unreacted orconcentration quenching may occur. Suitable proportions of zinc oxideand tantalum may be achieved for many applications by controlling thetantalum layer thickness to be about 10 percent of the initial thicknessof the zinc oxide layer. However, for low-voltage cathodoluminescentapplications (i.e. those employing low-energy electrons), the tantalumlayer thickness can be much less than 10 percent of the zinc oxide layerthickness, since the cathodoluminescence is a surface phenomenon. (Thelow voltage electron penetration and excitation takes place very nearthe phosphor surface--a few tens of nanometers for electrons below 500V). Also the spectral output can include the normal spectral output ofzinc oxide if the tantalum layer is kept thin enough so that theelectrons excite cathodoluminescence in both the new phosphor and theunderlying zinc oxide material. A tantalum thickness of 0.25 percent ofthe zinc oxide thickness (e.g. 2.5 nanometers of Ta for 1 micrometerZnO) has been successfully used. Another especially useful arrangementhad a film of Ta about 7.5--10 nanometers thick on about 200 nanometersof ZnO before annealing to form the phosphor in a very thin form.

An anneal (S4) of the layered materials is used to form the new phosphormaterial. A preferred way to anneal the zinc oxide/tantalum compositestructure is to heat the composite at 1200° C. for two minutes in aninert environment, such as an atmosphere of nitrogen gas. This iscommonly referred to as a "rapid thermal anneal" (RTA) or sometimes as a"rapid thermal process" (RTP). Wide ranges of effective temperatures andanneal times exist. Even a ten second anneal at 1200° C. is sufficientto substantially eliminate a green peak from the cathodoluminescencespectrum. A low temperature of 900 degrees centigrade and a hightemperature of 1250° C. have been successfully used. The times usedvaried from 10 seconds to 6 minutes. However, these are not limitations.Other annealing techniques are also effective such as a standard thermalanneal in a furnace tube. By way of example, 1100° C. for 90 minutes hasbeen an effective anneal. Laser annealing is another example of asuitable annealing process.

During the anneal, Ta combines with ZnO to some depth. In one sampleusing 1 micrometer thickness of ZnO and 30 nm of Ta and annealed at1100° C. in an Ar/O₂ atmosphere, Auger electron spectroscopy profilingshowed Ta present from the surface of the composite material to a depthof about 1/3 of the initial ZnO layer thickness, with a peak Taconcentration at about 150 nm below the surface. X-ray diffraction ofsuch compositions indicate that they include both Ta₂ Zn₃ O₈ and ZnO.The Ta₂ Zn₃ O₈ is believed to be responsible for the blue luminescenceobserved. According to a scientific paper by Kasper "DieKoordinationsverhaltnisse in Zinkniobat und -tantalat" in "Zeitschriftfur anorganische und aligemeine Chemie" Vol. 355, No. 1-1, pp. 1-11(Nov. 1967), another possible phase with lower molar ratios of Ta₂ O₅ toZnO is the phase Ta₂ ZnO₆. This phase was not observed in the X-raydiffraction patterns mentioned above. It will be apparent to thoseskilled in the art that zinc oxide is an example of a preferred materialand that other oxide materials can be used as suitable host bases.Similarly, those skilled in the art will readily understand that otherrefractory transition metals can be used to prepare other specific newphosphor materials similar to the composition described herein.

The phosphor described herein is preferably prepared as a thin filmphosphor. However, it will be evident to those skilled in the art thatother forms of the new phosphor can easily be made. For example, afterthe phosphor material is synthesized, it can be ground to a powder of adesired particle-size distribution, such as the powder phosphorscommonly used in cathode ray tubes (CRT). The powdered form can bedeposited onto a desired substrate by any of the known techniques, suchas settling (sedimentation), spraying, or electrophoresis.

EXAMPLE 2

In this example, the starting materials are powders ofcommercially-available P24 phosphor (ZnO:Zn) and tantalum pentoxide (Ta₂O₅). Normalized cathodoluminescence spectra for the starting materialand a phosphor product made in accordance with the present invention areshown in FIG. 5. The normal dominant cathodoluminescent emission peak410 of P24 phosphor occurs at about 505 nm. These starting materials aremixed in suitable proportions (ignoring the excess Zn in the P24phosphor) for the reaction:

    3ZnO+Ta.sub.2 O.sub.5 →Ta.sub.2 Zn.sub.3 O.sub.8.

The mixture is heated at an effective temperature (over about 900° C.),e.g. 1200° C. for a suitable time (e.g. 2 hours), resulting in ablue-light-emitting phosphor having a dominant peak 420 ofcathodoluminescent emission at about 400 nm. A secondary (lowerintensity) cathodoluminescent emission peak at about 493 nm is observedfrom both P24 (430) and in the Ta₂ Zn₃ O₈ phase (440).

EXAMPLE 3

In this example (illustrated by the flow chart of FIG. 4), the startingmaterials are ZnO and Ta₂ O₅. The starting materials zinc oxide (ZnO)and tantalum pentoxide (Ta₂ O₅) are mixed (S5) in proportions accordingto molecular weight for Ta₂ Zn₃ O₈ to form a mixture; water is added(S6) to the mixture to form a slurry; the slurry is ball milled (S7) anddried (S8) to provide a dry intermediate mixture, the dry intermediatemixture is heated (S9) at an effective temperature, equal to or greaterthan 900° C. in air or other ambient atmospheres; the resultant materialis granulated (S10) to a fine powder. The fine powder is either (1)fired (S11) at an effective temperature and time, e.g. 1200° C. orhigher for about 1 hour or more, to form Ta₂ Zn₃ O₈, or (2) mixed (S12)with a binder such as polyvinyl alcohol, pressed into a solid (S13), andheated (S14) at about 1200° C. or higher for about 1 hour or more. Thesolid can be reground (S15) and annealed (S16) if required, or used(S17) as a sputter target, or simply used as a powder phosphor. Otherorganic polymers, such as cellulose acetate butyrate, a polyalkylmethacrylate, a polyvinyl-n-butyral, a copoly-(vinyl acetate/vinylchloride), a copoly-(acrylonitrile/butadiene/styrene), a copoly-(vinylchloride/vinyl acetate/vinyl alcohol), or a mixture of such organicpolymers, may be used as the binder as alternatives to polyvinyl alcoholin step S12.

EXAMPLE 4

This example is an integrated fabrication technique using an etch stopthat is also at the same time a component of an in situ phosphor-formingprocess. The etch stop material is used both to prepare the newlow-voltage phosphor material and to define the depth of an opening in adisplay structure utilizing the phosphor. The etch stop solves theproblem of over-etching that frequently occurs in the fabrication offield-emission displays. The etch stop first protects the nascentphosphor layer and subsequently is also used in preparation of the newphosphor. The etch stop layer does not need to be removed. After ananneal, at least a portion of the etch stop is incorporated into thebase phosphor and modifies the composition of the base phosphor toproduce the new phosphor, as described above in Example 1. Examples ofapplications of this integrated phosphor preparation process, performedin situ, include phosphor plates, screens, panels, and display matrices,among others.

The etch stop material is typically comprised of a refractory metal.However, other suitable materials can be used. Different materials canbe used to modify the base phosphor selectively at different specificlocations. This selective disposition of different substances can beused to make different spectral outputs at different locations. Dopantscan be added to the etch stop material and, hence, are also incorporatedinto the base phosphor material to modify the spectral output. This maybe also (or alternatively) done at specific locations by depositing thedopant(s) selectively. The selective disposition of dopants can beperformed by ion implantation, for example.

The preferred fabrication method is described in detail below, withreference to FIG. 6, which is a flow chart illustrating the in situprocess. Steps of the process are denoted by references S20, S30, . . ., S90. A suitable substrate 20 is provided (S20). The substrate can be aconductive substrate, an insulating substrate, or an insulatingsubstrate with patterned conductive portions, for example. In step S30,a host phosphor 30 such as zinc oxide is deposited on the substrate. Alayer of a refractory metal 40 such as tantalum is then deposited (S40).If desired, the layer of refractory metal may be patterned (S50) byconventional patterning methods. A layer of insulator 50 is deposited(S60) over the refractory metal. The insulating layer is patterned (S70)to define an area for an opening 120. The insulator is etched (S80) tothe etch stop provided by the refractory metal, to form an opening whosedepth is precisely and automatically determined by the etch stop. Instep S90, the entire structure is annealed to form the new phosphormaterial 35.

It will be understood by those skilled in the art that this preferredprocess description is an example only and does not limit the presentinvention to the particular materials, process conditions, or processsequence mentioned above. For example, there are numerous effectiveanneal conditions and many other refractory metals, such as, but notlimited to, molybdenum, zirconium, titanium, and tungsten. There arealso many dopants that can be used to dope the new phosphor, such as,but not limited to, manganese and/or chromium and/or lanthanides.Furthermore, there are many different types of substrates that can beused depending on the application. These include, but are not limitedto, conductors, semi-conductors, insulators, and mixtures or compositesthereof Some suitable substrates are silicon, silicon oxide, siliconnitride, metallized silicon oxide, and glass.

The process just described may be usefully included and integrated as asubprocess in an overall process for fabrication of electronfield-emission-device displays (FED). This method discloses an etch stopwhich may be used in a trench etch process such as the etch process ofU.S. Pat. No. 5,618,216 to Potter, the entire disclosure of which isincorporated herein by reference. The etch stop material is also used tocreate the new low-voltage phosphor material of the present application.The etch stop solves the problem of over etching during the opening ortrench formation process. It first protects the phosphor layer, and itsubsequently participates in formation of the new phosphor. The etchstop layer does not need to be removed. After an anneal, it isincorporated into the phosphor layer and modifies the initial phosphorlayer's luminescence, particularly the chromaticity of itscathodoluminescence.

The etch stop material is typically comprised of a refractory metal.However, other suitable materials can be used. Different materials canbe used selectively to modify the base phosphor layer at specificlocations or pixel sites in order to fabricate a display emitting lightof more than one color, e.g. different colors for different pixels.Alternatively, dopants can be added to the etch stop material and,hence, also incorporated into the base phosphor layer material to modifythe spectral output. This may be done selectively at specific selectedlocations for creating different color pixels, e.g. by ion implantationafter annealing, followed by a re-anneal if necessary to incorporate thedopant.

The preferred overall process for fabrication of electronfield-emission-device displays is described in the following paragraphs,with reference to FIGS. 7a-7k and the flow chart of FIG. 8. The sideelevation cross-section views of FIGS. 7a-7k are not drawn to scale.Steps of the process are denoted by references S110, S120, etc. Forclarity, different step numbers are used within each process describedin this specification, but the person of ordinary skill will recognizethat several steps having different reference designations areequivalent. For example, the first step of the process to be describedbelow is step S110, providing a suitable substrate, which is equivalentto steps S1 and S20 described above. For the present example thesubstrate 20 may be a flat conductive substrate. Once a suitablesubstrate 20 is provided (S110), a host (base) oxide 30 such as zincoxide is deposited (S120, FIG. 7a). A layer 40 of a refractorytransition metal such as tantalum is deposited (S130) over the host(base) phosphor, FIG. 7b. If desired, the refractory metal layer 40 maybe patterned (S140) as shown in FIG. 7c. A first insulating layer 50 isdeposited (S150, FIG. 7d). If a lower gate element is desired, aconductive material 60 for a lower gate is deposited (S160, FIG. 7e) andpatterned (S170, not shown in FIGS. 7a-7k). A second insulating layer 70is deposited (S180, FIG. 7f). A thin film conductive emitter material 80is deposited (S190, FIG. 7g) and patterned (S200). (Patterning thatwould be visible in a plan view is not shown in the cross-sectionviews.) A third insulator 90 is deposited (S210, FIG. 7h). If an uppergate element is desired, a conductive material 100 for an upper gate isdeposited (S220, FIG. 7i) and patterned (S230) (not shown in FIGS.7a-7k). If a passivation layer is desired, a fourth insulating layer 110is deposited (S240, FIG. 7i). Separate contact holes, spaced apart fromeach other, are formed (S250) from the upper surface to conductivelayers such as the emitter layer and any gate layers, and conductivecontact material is deposited (S260), filling the contact holes andforming a contact layer. The contact layer is patterned (S270) toprovide contact patterns suitable to each type of contact (emitter,gate, etc.). The provision of contacts (S250-S270) is done in aconventional manner and is not shown in FIGS. 7a-7k. A trench mask 115is deposited (S280) and patterned (S290), as shown in FIG. 7i. A trenchopening 120 is etched (S300, FIG. 7j), which will stop on the etch stopprovided by the refractory metal 40 deposited in step S130. Only theleft side of opening 120 is shown in FIGS. 7j-7k; the right side is thesame. The structure formed is annealed (S310, FIG. 7k), for example in arapid thermal anneal in a nitrogen atmosphere for 10 seconds at 1200°C., to form phosphor layer 35 from base oxide layer 30 and refractorymetal etch stop layer 40. It will be understood by those skilled in theart that the foregoing preferred process description is by way ofexample only and does not limit the invention to the particularmaterials, process conditions, or process sequence mentioned. Aparticular useful variation is to form the phosphor 35 in situ byannealing or heating (S310) before etching (S300) the opening 120, sothat the phosphor itself serves as the etch stop that defines the depthof the opening 120. Dopant substances may be added in various waysbefore heating the other components, e.g. during preparation of a powderphosphor by mixing a quantity of dopant with the other startingmaterials, or co-deposited with the base oxide or the refractory metalduring the thin-film processes, for example. While amounts of dopants upto about 10 atomic percent may be used, preferred amounts are 5 atomicpercent or less. For low-electron-energy cathodoluminescenceapplications, any dopant used may be incorporated in only a thin surfacelayer on otherwise un-doped phosphor, as long as the doped surfaceregion thickness exceeds the small penetration depth of such low-energyelectrons.

Thus one aspect of the invention is a phosphor comprising, in atomicpercentages, 90% to 100% of a mixed metal oxide MxTyOz, wherein M is ametal selected from Zn, Sn, In, Cu, and combinations thereof, T is arefractory metal selected from Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, andcombinations thereof, and O is Oxygen, x, y, and z being chosen suchthat z is at most stoichiometric for MxTyOz, and also comprising 0% to10% of a dopant comprising a substance selected from a rare earthelement of the lanthanide series, Mn, Cr, and combinations thereof Otheraspects of the invention include methods for making such phosphors inthin-film form, in powder form, and in the form of a solid mass for useas a sputtering target. Another aspect of the invention is a particularmethod for forming such phosphors in situ on a substrate. A particularaspect of that method is an integrated etch-stop and phosphor-formingprocess, which is specially adapted for fabricating a field-emissiondisplay cell structure. Such field-emission display cell structures havea field-emission cathode and an anode comprising at least one of thephosphors of the present invention. They may also have one or more gateelements for controlling the electron current that flows from thecathode to the anode of the field emission devices when suitableelectrical bias voltages are applied. They may have more than onephosphor, and in particular may have red, green, and blue phosphorsselectively arranged. For example, each pixel site may have one anode ofeach color phosphor. The selective arrangement of various colorphosphors is done, in preferred embodiments, by selective deposition ofsuitable dopants. That dopant deposition may be done with appropriatemasking and chemical vapor deposition, or by selective ion implantation,for example. Yet another aspect of the invention is a display composedof one or more such field-emission display cell structures, each cellbeing formed by the processes described hereinabove.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions. Although specific embodiments of the present invention havebeen illustrated in the accompanying drawings and described in theforegoing detailed description, it will be understood that the inventionis not limited to the particular embodiments described herein, but iscapable of numerous rearrangements, modifications, and substitutionswithout departing from the scope of the invention, For example, numerouseffective anneal conditions may be used, and many refractory transitionmetals are available, such as tantalum, chromium, molybdenum, vanadium,niobium, zirconium, tungsten, hafnium, or titanium. There are manydopants that can be used to dope the new phosphors, such as manganese,chromium, and the rare earth elements of the lanthanide series. In onevariation of the sequence for instance, the base phosphor oxide can bedeposited (step S120) after the trench-forming step S300. Uses of thephosphors and associated processes and/or structures can include manydiverse applications, such as gamma-ray pinhole photography, forexample. Accordingly, the scope of the invention should be determinednot by the embodiments illustrated, but by the appended claims and theirequivalents.

Having described my invention, I claim:
 1. A process for making aphosphor, comprising the steps of:a) providing a substrate; b)depositing a first layer of a first metal oxide or of a compound capableof being converted by heat treatment to said first metal oxide, saidfirst metal oxide being selected from zinc oxide, tin oxide, indium-tinoxide, and copper oxide; c) depositing a second layer of a substancecontaining a refractory metal, said substance being selected fromtantalum, tantalum pentoxide, chromium, molybdenum, vanadium, niobium,zirconium, tungsten, hafnium, and titanium, at least one of said firstand second layers being disposed on said substrate; and d) heating saidfirst and second layers at an effective temperature for reacting atleast a portion of said first metal oxide and said refractory metal toform a mixed oxide thereof.
 2. A process for making a phosphor as inclaim 1, further comprising the step of adding a quantity of dopantselected from any of the lanthanide series of rare earth elements,manganese, chromium, and combinations thereof.
 3. A process for making aphosphor as in claim 2, wherein said phosphor has a spectral output andsaid dopant is selected fromi) manganese and terbium for providing agreen peak in said spectral output; and ii) europium for providing a redpeak in said spectral output.
 4. A process for making a phosphor as inclaim 1, wherein said first metal oxide comprises zinc oxide, saidrefractory metal comprises tantalum, and said heating is performed at atemperature of about 900° C. or higher.
 5. A process for making aphosphor as in claim 1, wherein said first metal oxide comprises tinoxide, said refractory metal comprises tantalum, and said heating isperformed at a temperature of about 900° C. or higher.
 6. A process formaking a phosphor as in claim 1, wherein said first metal oxidecomprises indium-tin oxide (ITO), said refractory metal comprisestantalum, and said heating is performed at a temperature of about 900°C. or higher.
 7. A process for making a phosphor as in claim 1, whereinsaid first metal oxide comprises cupric oxide, said refractory metalcomprises tantalum, and said heating is performed at a temperature ofabout 900° C. or higher.
 8. A process for making a phosphor as in claim1, wherein said heating step (d) is performed in an atmosphere includingan inert gas.
 9. A process for making a phosphor as in claim 8, whereinsaid inert gas is selected from nitrogen, argon, and mixtures thereof.10. A process for making a phosphor, comprising the steps of:a)providing a substrate; b) depositing a first layer of zinc oxide or of acompound capable of being converted by heat treatment to zinc oxide; c)depositing a second layer of a substance containing tantalum, at leastone of said first and second layers being disposed on said substrate;and d) annealing said first and second layers at an effectivetemperature for reacting at least a portion of said zinc oxide andtantalum to form a mixed oxide of zinc and tantalum.
 11. A process formaking a phosphor as in claim 10, wherein the step (b) of depositing afirst layer of zinc oxide is performed by depositing zinc oxideactivated by zinc in excess of a stoichiometric quantity.
 12. A processfor making a phosphor as in claim 10, wherein the step (b) of depositinga first layer of zinc oxide is performed by depositing a P24 phosphor.13. A process for making a phosphor as in claim 10, further comprisingthe step of adding a quantity of dopant selected from any of thelanthanide series of rare earth elements, manganese, chromium, andcombinations thereof.
 14. A process for making a phosphor as in claim13, wherein said phosphor has a spectral output and said dopant isselected fromi) manganese and terbium for providing a green peak in saidspectral output; and ii) europium for providing a red peak in saidspectral output.